Departmental Oral Examination (Thesis Title: “QED and X-ray Polarization from Neutron Stars and Black Holes”)

Abstract:
The emission from accreting black holes and neutron stars, as well as from the highly magnetized neutron stars called magnetars, is dominated by X-rays. For this reason, spectral and timing studies in the X-rays have been extremely successful in broadening our understanding of compact objects for the past few decades. Soon, a new observational window will open on compact objects: X-ray polarimetry. In this work, I explore how polarized light is generated in black-hole accretion disks, magnetar atmospheres and magnetospheres and in the accretion region of X-ray pulsars. In the different chapters, I show how the polarization signal is sensitive to several unknowns in our theoretical models: the geometry of accretion in X-ray pulsars, the strength and structure of the magnetic field threading accretion disks around black holes, the process of the non-thermal emission in magnetars. For this reason, the future X-ray polarimetry missions will be extremely helpful in constraining our theoretical models. Furthermore, the polarization emission will provide, for the first time, a test of one of the first theoretical prediction of quantum electrodynamics: vacuum birefringence. In this work I show how this effect, previously considered only for neutron stars, plays a crucial role for black holes as well.